Computer viruses and worms are often transmitted via electronic messages. An infectious message usually comes in the form of an e-mail with a file attachment, although other forms of infection are possible. Attackers have exploited many protocols that exchange electronic information, including email, instant messaging, SQL protocols, Hyper Text Transfer Protocols (HTTP), Lightweight Directory Access Protocol (LDAP), File Transfer Protocol (FTP), telnet, etc. When the attachment is opened, the virus executes. Sometimes the virus is launched through a link provided in the email. Virus or worm attacks can cause considerable damage to organizations. Thus, many anti-virus solutions have been developed to identify viruses and prevent further damage. Currently, most anti-virus products use virus signatures based on known viruses for identification. Such systems, however, often do not protect the network effectively during the time window between a virus' first appearance and the deployment of its signature. Networks are particularly vulnerable during this time window, which is referred to as “time zero” or “day zero”. For a typical anti-virus system to function effectively, it usually requires viruses to be identified, their signatures developed and deployed. Even after the system adapts after an outbreak, time zero threat can sometimes re-immerge as the virus mutates, rendering the old signature obsolete.
One approach to time zero virus detection is to use a content filter to identify and quarantine any message with a potentially executable attachment. This approach is cumbersome because it could incorrectly flag attachments in Word, Excel and other frequently used document formats even if the attachments are harmless, resulting in high rate of misidentification (also referred to as false positives). Furthermore, the approach may not be affective if the virus author disguises the nature of the attachment. For example, some virus messages ask the recipients to rename a .txt file as .exe and then click on it. Sometimes the virus author exploits files that were not previously thought to be executable, such as JPEG files. Therefore, it would be useful to have a better time zero detection technique. It would also be desirable if the technique  could detect viruses more accurately and generate fewer false positives.